Buck/boost method of voltage regulation for a permanent magnet generator (PMG)

ABSTRACT

The output voltage generated by permanent magnet generator (PMG) is regulated by controlling a buck/boost voltage applied to selected sub-coils within the PMG. The PMG includes a number of stator coils that are each divided into a number of sub-coils. A buck/boost voltage source generates a buck/boost voltage, and a controller connected to monitor the output voltage generated by the PMG selectively applies the buck/boost voltage to selected sub-coils based on the monitored output voltage. In this way, the controller is able to regulate the output voltage by selectively controlling the buck/boost voltage applied to the selected sub-coils.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. application Ser. No. 11/185,422entitled “BUCK/BOOST METHOD OF VOLTAGE REGULATION FOR A PERMANENT MAGNETGENERATOR (PMG),” now U.S. Pat. No. 7,224,147, filed on Jul. 20, 2005.

BACKGROUND OF INVENTION

A permanent magnet generator (PMG) is used to convert mechanical energy,usually rotational, to electrical energy. The typical PMG receivesmechanical energy from a prime mover. The prime mover may be, forexample, a gas turbine engine of an aircraft. The prime mover causes arotor located within the PMG to spin. Magnetic flux created by permanentmagnets located on the rotor cause an emf voltage to be generated instator windings. The accumulation of the voltage generated at each ofthese coils is provided as an output voltage to a load.

The output voltage generated by the PMG is dependent, in part, on thespeed of the prime mover as well as the overall impedance of the load.That is, a decrease in rotational velocity of the prime mover results ina decreased rotational velocity of the rotor, and a resulting decreasein the output voltage generated by the stator windings. An increase inrotation of the prime mover results in an increase of the output voltagegenerated by the coils in the stator. Likewise, a decrease in theimpedance of the load results in an increase in the output voltage ofthe PMG, and an increase in the impedance of the load results in adecrease in the output voltage of the PMG.

In many applications, variations in the output voltage of the PMG arenot acceptable. However, it is not always possible to precisely controlthe speed of the prime mover or the impedance of the load. In theseapplications, it would be desirable to be able to maintain the outputvoltage of the PMG despite variations in speed of the prime mover orimpedance of the load. In other applications, it is desirable to be ableto control the output voltage of the PMG without having to modify thespeed of the prime mover or the impedance of the load.

BRIEF SUMMARY OF INVENTION

In one aspect, the present invention is a permanent magnet generator(PMG) system for regulating an output voltage. The system includes a PMGhaving a rotor connectable to a prime mover, a stator having statorteeth, and stator coils wrapped around the stator teeth and divided intosub-coils. A buck/boost voltage is connected to selected sub-coils,wherein applying the buck/boost voltage to the selected sub-coilsincreases or decreases the output voltage. A controller monitors theoutput voltage and selectively applies the buck/boost voltage to thesub-coils based on the monitored output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a permanent magnet generatorsystem of the present invention for maintaining a constant outputvoltage.

FIG. 2 is a cross sectional view illustrating the geometry of apermanent magnet generator of the present invention.

FIG. 3 is a circuit diagram illustrating stator coils of the PMGconnected in a first configuration, in which the first sub-coils areconnected in series and second sub-coils are connected in series.

FIGS. 4A-4C are circuit diagrams illustrating the effect of applyingeither no voltage, a boosting voltage, or a bucking voltage to thesecond sub-coils of the PMG connected in the first configuration.

FIG. 5 is a circuit diagram illustrating stator coils of the PMGconnected in a second configuration, in which the first sub-coils areconnected in series with the second sub-coils.

FIG. 6 is a circuit diagram illustrating the effect of connecting thefirst and second sub-coils of the PMG in the second configuration.

FIG. 7 is a circuit diagram illustrating stator coils of the PMG dividedinto three sub-coils.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of permanent magnet generator voltageregulation system 10 of the present invention, including prime mover 12,permanent magnet generator (PMG) 14, three-phase output voltage VoA,VoB, and VoC, load 16, controller 18, switch array 19, and buck/boostvoltage source 20. Prime mover 12 is connected to PMG 14, supplying PMG14 with rotational, mechanical energy. PMG 14 converts mechanical energysupplied by prime mover 12 to electrical energy, which is supplied toload 16 and is illustrated as three-phase output voltage VoA, VoB, andVoC (collectively “output voltage Vo”).

There are two variables external to PMG 14 that affect output voltageVo. The first is the rotational velocity of prime mover 12. If primemover 12 rotates faster, then output voltage Vo will increase. If primemover 12 rotates slower, then output voltage Vo will decrease. Thesecond variable that affects output voltage Vo is the impedance of load16. If the impedance of load 16 increases, output voltage Vo provided byPMG 14 will decrease. If the impedance of load 16 decreases, outputvoltage 16 provided by PMG 14 will increase. In a number ofapplications, rotational velocity of the prime mover and impedance ofthe load are not easily controlled. The present invention provides asystem and method of maintaining a relatively constant output voltage Vodespite variations in rotational velocity of prime mover 12 andimpedance of load 16.

To maintain a constant output voltage Vo, controller 18 monitors outputvoltage Vo. If a change in output voltage Vo is detected, controller 18selectively operates a number of switches (shown in FIGS. 3, 5, and 7)located in switch array 19 to configure stator coils (shown in FIGS.2-7) to effectively regulate output voltage Vo. Each of the stator coilsis divided into two or more sub-coils. Depending on the output voltageVo detected, controller 18 controls switches in switch array 19 toconnect the sub-coils in one of many possible configurations to generatethe desired output voltage. Controller 18 is also responsible forcontrolling when and how buck/boost voltage Vbb is applied to thesub-coils of PMG 14. Buck/boost voltage Vbb, when applied to thesub-coils, can be used as the name suggests to either buck (decrease) orboost (increase) output voltage Vo.

There are several ways to generate buck/boost voltage Vbb. In each ofthese embodiments, buck/boost voltage is maintained at the samefrequency as output voltage Vo. The reasons for this are discussed inmore detail below, but without consistent frequency between buck/boostvoltage and output voltage Vo, constant voltage regulation cannot bemaintained. Depending on the operation (bucking or boosting) thebuck/boost voltage is maintained in phase with output voltage Vo(boosting) or 180 degrees out of phase with output voltage Vo (bucking).In one embodiment, a portion of output voltage Vo is feed back throughcontroller 19 and switch array 20 to the sub-coils of PMG 14. Thisarrangement ensures that frequency of buck/boost voltage Vbb and outputvoltage Vo are equal. In another embodiment, a second set of statorcoils (not shown), in addition to the set of stator coils located withinPMG 14 (shown in FIG. 2), is used to generate voltage from themechanical energy provided by prime mover 12. As prime mover 12 rotates,stator coils within PMG 14 as well as the second set of stator coilsgenerates voltage. Because the frequency of prime mover 12 is the samefor both the set of stators located within PMG 14 and the second set ofstators, the frequency of the voltage generated by PMG 14 and the secondset of stator coils is equal. In both of these embodiments, buck/boostvoltage Vbb is generated in phase with output voltage Vo. By switchingthe input leads connecting buck/boost voltage Vbb to selected sub-coils,the phase of buck/boost voltage can be changed by 180 degrees. Thus,during a boosting operation, buck/boost voltage Vbb generated by one ofthe methods described above is delivered to the selected sub-coils inphase with output voltage Vo. During a bucking operation, buck/boostvoltage Vbb generated by one of the methods described above is deliveredto the selected sub-coils 180 degrees out of phase with output voltageVo.

FIG. 2 shows an embodiment of permanent magnet generator 14 of thepresent invention, including stator 22, rotor 24, and axis 26 connectingrotor 24 to prime mover 12 (shown in FIG. 1). Rotor 24 includes aplurality of poles P1-P24, making this a twenty-four pole rotor. Stator22 includes a plurality of stator slots S1-S36, making this a thirty-sixslot stator. Each adjacent set of slots, for instance slots S1 and S2,form a stator tooth upon which a coil or winding is wrapped (in thiscase, coil CA1). Because the output of PMG 14 is three phase power,there are three electrically separate sets of coils.

For ease of illustration, FIG. 2 shows only the set of coils CA1-CA12responsible for output voltage VoA (shown in FIG. 1 as one of the threephases of power). Coil CA1 is wrapped around the stator tooth locatedbetween stator slots S1 and S2. Coil CA2 is wrapped around the statortooth located between stator slots S4 and S5. Coil CA3 is wrapped aroundthe stator tooth located between stator slots S7 and S8, and so on. Inorder to maximize the efficiency, coils CA1-CA12 are spaced equallyaround stator 22. The equal spacing of coils CA1-CA12 and aslot/pole/phase ratio of 0.5 (36 slots/24 poles/3 phases) results ineach coil CA1-CA12 being exposed to equal magnitude and direction ofmagnetic flux generated by poles P1-P24 such that output voltage VoA ismaximized. Two other sets of coils (not shown) are wound in similarfashion around the remaining stator teeth located between slots S1-S36to create the other two phases of power, VoB and VoC. As shown in FIGS.3-7 and discussed above, each coil CA1-CA12 (as well as the coils usedfor generated output voltage VoB and VoC) is divided into sub-coils,which may be connected in a number of configurations to either increaseor decrease output voltage Vo.

Rotation provided by prime mover 12 is transferred to rotor 24 via axis26. Thus, prime mover 12 causes rotor 24, and magnetic poles P1-P24located on rotor 24, to spin. Voltage in the number of coils CA1-CA12 iscreated by the rotation of magnetic poles P1-P24, and the resultingmagnetic flux seen by coils CA1-CA12. Each adjacent pole is necessarilyof a different polarity. For instance, if magnetic pole P2 is a magneticnorth pole, then both magnetic poles P1 and P3 are magnetic south poles.Magnetic flux generated by adjacent poles travels in part through thenumber of coils C1A-C12A (as well as those coils not shown) withinstator 22. As rotor 24 spins, magnetic flux through coils CA1-CA12varies, resulting in emf voltage being generated in each of the numberof coils CA1-CA12. As shown in FIGS. 3-7, coils CA1-CA12 are eachdivided into sub-coils. Depending on the connection of the varioussub-coils, output voltage VoA can be regulated as desired. Outputvoltage Vo generated by the number of coils is described in thefollowing equation, assuming the output voltage is not connected to aload.Vo=4.44*(Frequency)*(#ofTurns)*(MagneticFlux)*(Area*10⁻⁸)  EQ. 1

Equation 1 illustrates the variables that affect output voltage Vo. Eachof the variables is directly related to voltage, thus as frequency,number of turns, magnetic flux or area increase, so does output voltage16. Likewise if any of these variables decrease, so does output voltage16. Frequency is related to the speed at which prime mover 12 rotates,and area refers to the area cross section of each of the number of coilsCA1-CA12. The frequency of the prime mover is external to PMG 14 and isnot directly controlled in the present invention. Likewise, area is alsokept constant in the present invention. This leaves number of turns andmagnetic flux as the remaining variable that can affect of outputvoltage Vo.

Number of turns relates to the number of turns of wire making up eachcoil CA1-CA12. In one aspect of the present invention, controller 18selectively modifies how the sub-coils are connected. This allowscontroller 18 to adjust the number of turns contributing to createoutput voltage Vo. Likewise, magnetic flux refers to the density ofmagnetic flux within each coil CA1-CA1 2. As shown in FIGS. 3-7, thepresent invention also selectively modifies the density of magnetic fluxin each coil by selectively applying buck/boost voltage Vbb to thesub-coils in order to modify output voltage 16. FIGS. 3-7 illustrate anumber of ways in which coils C1-C12, as well as their counterparts inthe other two phases, can be connected to vary the output voltagegenerated.

FIG. 3 shows coils CA1-CA12, each coil divided into first sub-coils CA1a-CA12 a and second sub-coils CA1 b-CA12 b. For instance, coil CA1 isdivided into first sub-coil CA1 a and second sub-coil CA1 b, and coilCA2 is divided into first sub-coil CA2 a and second sub-coil CA2 b. Tomaintain equal voltage generation between each of the coils CA1-CA12depending on the particular configuration selected, the number of turnsof each of the plurality of first sub-coils CA1 a-CA12 a is equal, andthe number of turns of each of the plurality of second sub-coils CA1b-CA12 b is equal. For instance, in this embodiment, the ratio of turnsbetween first sub-coils CA1 a-CA12 a and coils CA1-CA12 is 0.6:1. Thatis, coils CA1-CA12 are divided such that first sub-coils CA1 a-CA12 acontain 60% of the turns, and second sub-coils CA1 b-CA12 b contain theremaining 40% of the turns. For the sake of simplicity, this embodimentshows each coil divided into two sub-coils.

A plurality of switches SW1-SW36 (generally SW) allows first and secondsub-coils to be connected in a number of configurations. In theexemplary embodiment shown in FIG. 3, switch SW1 is connected to firstsub-coil CA1 a, and switches SW2 and SW3 are connected to secondsub-coil CA1 b. Likewise, switch SW4 is connected to first sub-coil CA2a, and switches SW5 and SW6 are connected to second sub-coil CA2 b. Eachswitch SW selects between one of two possible connections. Theconnections associated with each switch SW are labeled in FIG. 3 andFIG. 5 with a “1” or a “2”. In the first configuration, shown in FIG. 3,switches SW are set to the first position labeled with a “1”. In thesecond configuration, shown in FIG. 5, switches SW are set to the secondposition labeled with a “2”. By setting switches SW to the firstposition, first sub-coils CA1 a-CA12 a are connected in series with oneanother. For example, first sub-coil CA1 a is connected in series withfirst sub-coil CA2 a via switch SW1. Likewise, if switches SW are in thefirst position, second sub-coils CA1 b-CA12 b are connected in serieswith one another. For example, second sub-coil CA1 b is connected inseries with second sub-coil CA2 b via switch SW3 and switch SW5. Abuck/boost voltage source Vbb is also connected to the series of secondsub-coils CA1 b-CA12 b. As shown in FIG. 3, buck/boost voltage Vbb isconnected to second sub-coil CA1 b via switch SW2 and to second sub-coilCA12 b via switch SW 36.

The output voltage generated when the plurality of switches SW are inthe first position is dependent on the application of buck/boost voltageVbb. The buck/boost voltage can be applied in three ways; no buck/boostvoltage is applied (effect illustrated in FIG. 4A), buck/boost voltageapplied such that magnetic flux generated in second sub-coils increasesmagnetic flux in first sub-coils (effect shown in FIG. 4B), andbuck/boost voltage applied such that magnetic flux generated in secondsub-coils decreases magnetic flux in first sub-coils (effect shown inFIG. 4C). Setting switches SW to the second position (shown in FIG. 5)connects first sub-coils CA1 a-CA12 a and second sub-coils CA1 b-CA12 bin series with one another, increasing the total number of turns beingutilized to generate output voltage VoA, resulting in an increase ofoutput voltage VoA per equation 1. Each of these configurations isdiscussed in more detail below.

FIG. 4A shows the effect of applying no buck/boost voltage Vbb to theplurality of second sub-coils CA1 b-CA12 b. Because no buck/boostvoltage Vbb is applied to the plurality of second sub-coils CA1 b-CA12 bconnected in series with one another, no magnetic flux is generated insecond sub-coils CA1 b-CA12 b. With no magnetic flux generated in secondsub-coils CA1 b-CA12 b, they have no effect on the generation of outputvoltage created by first sub-coils CA1 a-CA12 a. Magnetic flux generatedby the plurality of poles P1-P24 (shown in FIG. 2) crossing throughfirst sub-coils CA1 a-CA12 a results in generation of voltage in each ofthe first sub-coils. Because each of the first sub-coils CA1 a-CA12 a isconnected in series and the magnetic flux passing through each sub-coilis oriented in the same direction in each of the sub-coils, outputvoltage VoA generated is the combined total of the voltage generated ateach of the first sub-coils CA1 a-CA12 a.

This configuration of sub-coils, along with the absence of any type ofbuck/boost voltage Vbb is labeled as normal operation. From thisconfiguration, output voltage 16 can be regulated despite variations ineither the speed of prime mover 12 (shown in FIG. 1) or in the impedanceof load 16 (shown in FIG. 1). That is, output voltage Vo can beincreased or decreased as necessary to offset increases or decreasescaused by outside variables.

FIG. 4B shows an embodiment in which switches SW are again connected inthe first position, resulting in first sub-coils CA1 a-CA12 a beingconnected in series, and second sub-coils CA1 b-CA12 b being connectedin series with buck/boost voltage Vbb. However, in FIG. 4B buck/boostvoltage Vbb is applied to second sub-coils CA1 b-CA12 b to create aboosting effect on output voltage VoB. The buck/boost voltage Vbb isapplied in phase with output voltage VoA, such that magnetic fluxgenerated in second sub-coils CA1 b-CA12 b is in the same direction asmagnetic flux generated by rotor 24 (shown in FIG. 2).

As rotor 24 spins along with magnetic poles P1-P24, magnetic flux seenby coils CA1-CA12 changes direction, resulting in the generation ofalternating current power. In order for buck/boost voltage Vbb toconsistently increase the magnetic flux in sub-coils CA1 a-CA12 a,buck/boost voltage Vbb must be maintained at the same frequency asoutput voltage VoA. Various ways of maintaining the proper frequency ofbuck/boost voltage Vbb were discussed above.

The additive nature of flux in first sub-coils CA1 a-CA12 a and secondsub-coils CA1 b-CA12 b is shown by the arrows located adjacent eachsub-coil in FIG. 4B. The magnetic flux generated in second sub-coils CA1b-CA12 b increases the net amount of magnetic flux in first sub-coilsCA1 a-CA12 a, and therefore increases output voltage VoA. It isimportant to note, that while first sub-coil CA1 a and second sub-coilCA1 b are shown as electrically separate elements in FIGS. 3-7, they areeach sub-parts of coil CA1, which is wrapped around a single statortooth. Therefore, first sub-coil CA1 a and second sub-coil CA1 b arecoupled together by the stator tooth coil CA1 is wrapped around.Magnetic flux generated in second sub-coils CA1 b-CA12 b influences theamount of flux in first sub-coils CA1 a-CA12 a. As shown in Equation 1above, increasing the magnetic flux seen by first sub-coil CA1 a resultsin an increase in voltage generated by first sub-coil CA1 a. Therefore,the output voltage created by the switch configuration and buck/boostvoltage Vbb shown in FIG. 4B results in an increase in output voltageVoA with respect to the output voltage generated with respect to FIG.4A. The number of coils responsible for producing output voltage VoA isthe same in FIGS. 4A and 4B, however, additional magnetic flux isgenerated in first sub-coils CA1 a-CA12 a in the embodiment shown inFIG. 4B, resulting in greater output voltage VoA. Output voltages VoBand VoC generated by other coils (not shown) would be the same.

FIG. 4C illustrates the effect of connecting switches SW in the firstposition and applying a bucking voltage to second sub-coils CA1 b-CA12b. The buck/boost voltage Vbb is applied such that magnetic fluxgenerated in second sub-coils CA1 b-CA12 b is in the opposite directionas the magnetic flux generated by rotor 24 (shown in FIG. 2) and seen byfirst sub-coils CA1 a-CA12 a. Again, this requires buck/boost voltageVbb be maintained at the same frequency as output voltage Vo. However,in order for buck/boost voltage Vbb to decrease the magnetic flux seenin first sub-coils CA1 a-CA12 a, the phase of buck/boost voltage Vbb isset to be 180° out of phase (i.e., inverted) with output voltage VoA. Inone embodiment, the phase of buck/boost voltage Vbb is modified byswitching the leads connecting buck/boost voltage Vbb to secondsub-coils CA1 b-CA12 b. The directional arrows adjacent to first andsecond sub-coils illustrate how magnetic flux generated in secondsub-coils CA1 b-CA12 b opposes magnetic flux seen in first sub-coils CA1a-CA12 a. The magnetic flux generated in second sub-coils CA1 b-CA12 bdecreases the amount of magnetic flux in first sub-coils CA1 a-CA12 a,and therefore decreases output voltage VoA. As discussed above, magneticflux generated in second sub-coil CA1 b is transmitted along the statortooth to influence first sub-coil CA1 a. By decreasing the magnetic fluxin first sub-coils CA1 a-CA12 a using buck/boost voltage Vbb to createopposing magnetic flux in second sub-coil CA1 b-CA12 b, output voltageVoA is decreased. As shown in Equation 1 above, decreasing the magneticflux seen by first sub-coil CA1 a results in a decrease in voltagegenerated by first sub-coil CA1 a. Therefore, output voltage VoA isdecreased with respect to the output voltage generated with respect toFIG. 4A. The number of coils responsible for producing output voltageVoA is the same in FIGS. 4A-4C, however, less magnetic flux is generatedin first sub-coils CA1 a-CA12 a in the embodiment shown in FIG. 4C,resulting in a decrease in output voltage VoA.

Although the present invention can be used to maintain a constant outputvoltage Vo, the mechanics of how this is done are more easily understoodif we assume the impedance of load 16 and the rotational velocity ofprime mover 12 remain constant while the effect of differentconfigurations on output voltage Vo are explored. By way of example,assume coils CA1-CA12 (without taking into account the configuration ofthe sub-coils) are capable of generating 100 volts (rms) at outputvoltage Vo at the current frequency of prime mover 12 and impedance ofload 16. Further, assume first sub-coils CA1 a-CA12 a account for 80% ofthe windings making up coils CA1-CA12 (first sub-coil to coil turn ratioof 0.8:1). With these assumptions, if first sub-coils CA1 a-CA12 a andsecond sub-coils CA1 b-CA12 b are connected in the first configurationand no buck/boost voltage Vbb is applied as shown in FIG. 4A, then firstsub-coils CA1 a-CA12 a will generate 80 volts (rms) at output voltageVoA. If buck/boost voltage Vbb is applied to second sub-coils CA1 b-CA12b as shown in FIG. 4B, causing an increase in magnetic flux seen byfirst sub-coil CA1 a-CA12 a, then 100 volts will be generated at outputvoltage VoA. Likewise, if buck/boost voltage Vbb is applied to secondsub-coils CA1 b-CA12 b as shown in FIG. 4C, causing a net decrease inmagnetic flux seen by first sub-coil CA1 a-CA12 a, then 60 volts will begenerated at output voltage VoA.

FIG. 5 shows an embodiment in which switches SW1-SW36 are connected in asecond configuration (each switch is moved to the position labeled “2”),resulting in first sub-coils CA1 a-CA12 a and second sub-coils CA1b-CA12 b being connected together in series. That is, first sub-coil CA1a is connected in series with second sub-coil CA1 b via switches SW1 andSW2. Second sub-coil CA1 b is connected in series with the next firstsub-coil CA2 a via SW3. Buck/boost voltage Vbb is not connected toanything in this configuration. Because each of the sub-coils isconnected in series, this configuration operates essentially just ascoils CA1-CA12. That is, first sub-coil CA1 a and second sub-coil CA1 boperate as coil CA1. Similarly, first sub-coil CA2 a and second sub-coilCA2 b operate as coil CA2.

FIG. 6 shows the effect of connecting first sub-coils CA1 a-CA12 a andCA1 b-CA12 b in series. In this configuration, although no magnetic fluxis being created by buck/boost voltage Vbb in the number of secondsub-coils CA1 b-CA12 b, the output voltage VoA generated by PMG 14 isincreased because of the additional number of coil turns added. RecallEquation 1 stated that output voltage Vo is directly related to numberof turns, as the number of turns increases, so does output voltage VoA.In the embodiment shown in FIG. 6, second sub-coils CA1 b-CA12 b areconnected in series with first sub-coils CA1 a-CA12 a, increasing thenumber of coil turns used to generate output voltage VoA. Increasing thenumber of coil turns results in an increase in output voltage VoA.

Using the numerical example provided above, assuming coils CA1-CA12 arecapable of generating 100 volts (rms) at the current frequency of primemover 12 and current impedance of load 16. By connecting first sub-coilsCA1 a-CA12 a in series with CA1 b-CA12 b, the number of windings beingused to generate output voltage VoA is equal to the number of windingsmaking up coils CA1-CA12, resulting in 100 volts (rms) being generatedat output voltage VoA.

As shown in FIGS. 3-6, output voltage Vo can be altered or maintained byadjusting the magnetic flux flowing through the sub-coils responsiblefor generating output voltage Vo, or by adjusting the number of windingsused to generate output voltage Vo. However, as shown in FIGS. 3-6,dividing coils into two sub-coils allows limited flexibility inadjusting output voltage Vo. By increasing the number of sub-coilswithin each coil CA1-CA12, the more finely output voltage VoA can betuned.

For instance, FIG. 7 shows coils CA1-CA12 divided into three sub-coils,first sub-coils CA1 a-CA12 a, second sub-coils CA1 b-CA12 b, and thirdsub-coils CA1 c-CA12 c. For the sake of simplicity, sub-coils are shownconnected to switching array 19 rather than to individual switches.Buck/boost voltage Vbb is applied to selected sub-coils through switcharray 19 as well. This allows buck/boost voltage Vbb to be applied tomore than one set of sub-coils. For the sake of simplicity, we againassume that each coil CA1-CA12 is composed of 100 turns. For examplethen, first sub-coil CA1 a-CA12 a is composed of 60 turns, and secondsub-coil CA1 b-CA12 b is composed of 30 turns, and third sub-coil CA1c-CA12 c is composed of the remaining 10 turns.

The mechanics regarding bucking or boosting voltage by application ofbuck/boost voltage Vbb as well as bucking or boosting voltage by addingor subtracting the number of sub-coils (and thus number of turns) usedto generate output voltage Vo remain the same as in the examplesdiscussed above. The difference lies in the increased number ofconfigurations possible, resulting in an increase in tuning capabilityof output voltage Vo.

For instance, if only first sub-coils CA1 a-CA12 a are connected inseries, and no buck/boost voltage Vbb is applied, then in our numericalexample first sub-coils CA1 a-CA12 a would generate 60 volts (rms) atoutput voltage VoA. By selectively connecting either second sub-coilsCA1 b-CA12 b or third sub-coils CA1 c-CA12 c and applying buck/boostvoltages Vbb, output voltage VoA can be tightly controlled. As shown inFIG. 7, because buck/boost voltage Vbb is applied to different sub-coilsdepending on the situation, buck/boost voltage Vbb is connected toswitches within switch array 19, allowing controller 18 to select thesub-coils to which buck/boost voltage Vbb is applied to in addition tocontrolling the phase in which buck/boost voltage Vbb is applied. Anumber of examples are provided in Table 1 below that illustrate howconnecting the sub-coils in different configurations affects outputvoltage VoA, although in operation, the present invention can connectthe sub-coils in different configurations to maintain an essentiallyconstant output voltage VoA despite speed and load changes.

TABLE 1 Desired Output Configuration of Sub- Voltage VoA CoilsBuck/Boost Voltage 20 volts (rms) 1^(st) sub-coils connected Buckingvoltage alone in series. applied to series 2^(nd) and 3^(rd) sub-coilsconnection of 2^(nd) and connected in series with 3^(rd) sub-coils. oneanother 30 volts (rms) 1^(st) sub coils connected Bucking voltage alonein series applied to 2^(nd) sub- 2^(nd) sub-coils connected coils. alonein series 3^(rd) sub-coils not used. 40 volts (rms) 1^(st) and 3^(rd)sub-coils Bucking voltage connected in series with applied to 2^(nd)sub- one another. coils. 2^(nd) sub-coils connected alone in series 50volts (rms) 1^(st) sub coils connected Bucking voltage alone in series.applied to 3^(rd) sub- 3^(rd) sub-coils connected coils. alone inseries. 2^(nd) sub-coils not used. 60 volts (rms) 1^(st) sub coilsconnected No buck/boost alone in series. voltage applied. 2^(nd) and3^(rd) sub-coils not used. 70 volts (rms) 1^(st) and 3^(rd) sub-coils Nobuck/boost connected in series with voltage applied. one another. 2^(nd)sub-coils not used. 80 volts (rms) 1^(st) and 2^(nd) sub-coils Buckingvoltage connected in series with applied to 3^(rd) sub- one another.coils. 3^(rd) sub-coils connected alone in series. 90 volts (rms) 1^(st)and 2^(nd) sub-coils No buck/boost connected in series with voltageapplied. one another. 3^(rd)sub-coils not used. 100 volts (rms)  1^(st),2^(nd) and 3^(rd) sub-coils No buck/boost connected in series withvoltage applied. one another.

As seen from this example, dividing coil CA1-CA12 into three coilsprovides greater flexibility in controlling output voltage VoA andtherefore in maintaining an essentially constant output voltage VoA (andlikewise, in output voltage VoB and VoC).

A voltage regulation system and method has been described in whichstator coils are sub-divided into at least two sub-coils. In otherembodiments, stator coils may be sub-divided into more than twosub-coils, allowing for a greater number of configuration options. Themethod of regulation employs a controller which selectively controls anumber of switches to connect the number of sub-coils in differentconfigurations (allowing the number of turns used to generated outputvoltage to be varied), as well as selectively applying buck/boostvoltage to increase or decrease the magnetic flux seen by the set ofcoils responsible for generating the output voltage. A controller isresponsible for configuring the sub-coils as well as applying anappropriate buck/boost voltage such that output voltage from the PMG isregulated to a desired value. For instance, if output voltage beginsincreasing due to an increase in the speed of the prime mover ordecrease in the impedance of a load connected to the PMG, the controllerconfigures the number of sub-coils and the buck/boost voltage todecrease the output voltage of PMG. In other embodiments, it may bedesirable to selectively increase or decrease the output voltage of thePMG, rather than maintain a constant voltage level.

1. A permanent magnet generator (PMG) system for regulating an outputvoltage, the system comprising: a permanent magnet generator (PMG),comprising: a rotor; a stator having stator teeth; and stator coilswrapped around the stator teeth and divided into sub-coils, wherein afirst set of sub-coils is connected in series to generate the outputvoltage; a buck/boost voltage source for producing a buck/boost voltage;and a controller connected to monitor the output voltage and to applythe buck/boost voltage to selected sub-coils not connected to the firstset of sub-coils based on the monitored output voltage.
 2. The system ofclaim 1, wherein the controller causes the buck/boost voltage to beapplied in phase with the output voltage to increase the output voltage.3. The system of claim 1, wherein the controller causes the buck/boostvoltage to be applied 180 degrees out of phase with the output voltageto decrease the output voltage.
 4. The system of claim 1, including:switches connected to the sub-coils and controlled by the controller toallow the sub-coils to be connected in a plurality of differentconfigurations to control the output voltage.
 5. The system of claim 1,wherein the sub-coils within each of the stator coils include a numberof turns, each sub-coil within one of the stator coils having adifferent number of turns.
 6. The system of claim 1, wherein thebuck/boost voltage source generates the buck/boost voltage at afrequency equal to a frequency of the output voltage.
 7. A permanentmagnet generator system, the system comprising: a permanent magnetgenerator that includes a number of stator coils divided into sub-coils,wherein selected sub-coils are connected to generate an output voltage;and a controller connected to monitor the output voltage generated bythe PMG and to apply a buck/boost voltage to sub-coils that are notconnected to generate the output voltage to either increase or decreasethe output voltage based on the monitored output voltage.
 8. The systemof claim 7, wherein the controller applies a buck/boost voltage in phasewith the monitored output voltage if the monitored output voltage isbelow a desired level.
 9. The system of claim 7, wherein the controllerapplies a buck/boost voltage 180 degrees out of phase with the monitoredoutput voltage if the monitored output voltage is above a desired level.10. The system of claim 7, further including: a plurality of switcheswithin a switch array that are controlled by the controller toselectively connect the sub-coils together in one of a plurality ofconfigurations based on the monitored output voltage.
 11. The system ofclaim 10, wherein the controller controls the plurality of switches toconnect selected sub-coils in series with the sub-coils connected togenerate the output voltage.
 12. The system of claim 10, wherein thecontroller applies a buck/boost voltage, to selected sub-coils that arenot connected to generate the output voltage within a particularconfiguration to either increase or decrease the output voltage.